While the Sale doesn't start till the 22nd (so add credit to your Steam Wallet!), the event's Steam Cards are now available via crafting badges.

When you craft a badge you are usually given an emoticon and wallpaper for the game and a random coupon. Instead of the coupon you will now receive a "Mysterious X" card. When the Sale goes live on the 22nd these Mysterious cards will be replaced with the Holiday event cards and will then be able to craft a Holiday Sale Badge (these have NO level limit).